What feeds the (usually) blue wire that connects to one of the field terminals?
The ignition circuit. The other field terminal connects to the regulator green (usually) wire.
Blue wire(one going to the regulator) is battery power(key on), The green is the way the regulator "regulates" the alternator output. This is relatively low amperage and the regulator varies voltage to the field to control output.
The blue wire is the IGNITION ONE circuit.
That's how the factory labeled it, that's how the OP referred to it. You can call it a ham sandwich if you want, but no one's going to have it for lunch.
